For low levels of detection, bioassays usually require labels or tags which connect the recognition process to a transduction mechanism. Such labels include radioactive compounds, enzymes and light emitting materials. Radioactive labels are useful in that they offer a low level of detection, but they require expensive detectors, present a potential radiohazard, and can be unstable. In comparison, enzymatic labels are more stable, less hazardous, and less expensive, but are not as sensitive. Light emitting labels, though sensitive, require specialized detectors and labels, which are bulky and/or expensive.
In recent years there has been an increasing interest in magnetic labels for chemical and bioanalysis, as exemplified by the interest in immunomagnetic separation technology, which is a proven method for such tasks as monitoring parasites in raw surface water. In that particular example, the requirements of parasite filtration, concentration, separation, and monitoring require bulky instrumentation and manual operation. One such example is described in Kriz, C. B.; Radevik, K.; Kriz, D. “Magnetic Permeability Measurements in Bioanalysis and Biosensors,” Anal. Chem. 1996, 68, 1966, in which a ferromagnetic sample is placed in a container which in turn is placed in a measuring inductor electrically connected in a bridge sensing circuit.
There have been investigations that have demonstrated the feasibility of magnetic detection concepts as applied to biomolecules. Insofar as applicants are aware, that work has been limited to attempts to utilize the high sensitivity of giant magnetoresistive sensors (GMRs) for antibody and DNA detection through immobilization of capture molecules on GMR surfaces. These investigations recognize the high sensitivity of giant magnetoresistive sensors for antibody and DNA detection, but require the mechanism of immobilization of capture molecules on GMR surfaces. The capture molecules bind the magnetically labeled target analyte resulting in a change in the GMR resistance. This approach, which requires recognition specificity of binding between the capture and target molecules, can suffer from non-specific interactions that will affect the GMR reading and consequently compromise the level of detection. It also requires the selective binding of the analyte directly onto the surface of the GMR. U.S. Pat. No. 5,981,297 discloses the binding to a giant magnetoresistive sensor of binding molecules which are capable of immobilizing the target molecules. The output of the devices is then a measure of the number of analyte or target molecules bound to the cells of the sensor. In this arrangement, the goal is for rapid detection stated to be on the order of 15–30 minutes per assay.